Automatic information reading from bank credit cards, transportation fare cards, and like objects is rapidly gaining acceptance as the society moves towards a "cashless" money system. The information is usually placed, visibly or invisibly, on an article and read by mechanical means, by light-detection means or by magnetic-sensing means. For example, the so-called magnetic-strip credit cards or fare cards are particularly attractive because of their ease of manufacture. Unfortunately, this type of card has been demonstrated to be easily counterfeited. Despite this known fact, the convenience of the "magnetic strip" approach has caused the American Bank Association to pick the magnetic strip as its standard for bank credit plans.
In other magnetic recording systems, it is also desirable to have some means of building more security into the system. For example, knowledge of a code key would be useful in screening access to information stored on magnetic tape. This code key might be mechanical or electrical according to known procedures. But, such systems would not adequately discriminate between persons who are allowed access to some tapes but not allowed to retrieve information on other tapes. It would be convenient if each package of information, say each magnetic tape in a given library, held a code signal which had to be recognized by the retriever before being erased to reveal the primary information on the tape. It would be further convenient if the code could be changed from time to time as the authorized personnel change.
In general then, it would be convenient to provide magnetic recording media capable of carrying two independent signals in the same recording area and, advantageously as powders within the same matrix resin.
Previously it has been known, or at least alleged to have been known, to make multi-layer bimodal magnetic tapes using thin films of plated metal. Such recording media, disclosed in U.S. Pat. Nos. 3,328,195 and 3,219,353 to May and Prentky, give little or no evidence that any high performance bimodality was achieved. In any event, manufacture of the May and Prentky products require relatively expensive, multi-layer construction and also utilize metallic films, the formulation and design of which are not only very complex and expensive, but depend for their magnetization upon different considerations, e.g. the controlled movement of domain walls, than do magnetic particles dispersed in a resin matrix. Moreover, such films are not readily varied as to composition and are not believed as susceptible to enhancement of recording properties by use of bias currents because eddy currents seem to be induced, possibly because of the relatively high conductivity of metal films.
Other deficiencies in the bimodal materials of Prentky and May are the necessity of using very large coercivity differences. Trimodal or quatramodal media become economically and technically impractical using their processes and product.
Moreover, it appears that Prentky and May had no idea that materials, or at least those exhibiting substantial shape anisotropy could be used in polymodal materials. The Prentky and May materials exhibit little or no shape anisotropy although they exhibit a great deal of magnetic interaction because of the nature of domain-wall-modulated magnetic recording.
It is further noted that in an article entitled "The Effect of Particle Interaction on the Coercive Force of Ferromagnetic Micropowders" (Proceedings of the Royal Society, A, 232 (1189), pages 208-226, 1955), Wohlfarth discloses that "the change of coercive force with packing is negligible for powders controlled by magnetocrystalline anisotropy . . . but is significant for particles controlled mainly by shape anisotropy." This statement is related to the present invention only by a hindsight search for a theory by which to further explain some of the unexpected and advantageous attributes of the products of the invention.
In further hindsight review of art bearing some relationship to fields relating in some superficial structural or utilitarian aspect to the invention, the following art was located:
U.S. Pat. No. 3,601,913 to Pollock suggests a device utilizing a mixture of high and low coercivity materials. The utility of this system depends on the detection of magnetic voids caused by mechanical displacement of magnetic powder bearing surfaces. Pollock also suggests that use of such mixtures will make card counterfeiting more difficult. But Pollock uses his mixture only to complicate the manipulative acts of duplicating a card; the individual components serve no distinct functions except in instances wherein they are at least partly non-congruent so that they yield a visually-identifiable pattern or a geometrically distinct identifying pattern formed of one of the magnetic powders. In general, the powders simultaneously resond to the field as a single-population powder would.
Moreover, the chromium oxide and iron oxide mixture of the Pollock patent would be inoperable in forming combinations for use in such processes as are to be described below.
U.S. Pat. No. 3,790,754 to Black teaches use of adjacent ferromagnetic coatings to achieve a multi-modal, but relatively insecure magnetic recording system. The system is generally similar to Pollock's system.
In recently-issued U.S. Pat. No. 3,761,311 to Perrington et al, there is described a dual layer magnetic tape, each layer having different nominal coercivities. It is believed that such a tape is sold under the trade designation Scotch Brand C-60 Cassette Cobalt-Energized High-Energy Type (Catalog Number S-C-60ME) by 3M Company. That tape is not bimodal, probably because magnetic interaction substantially prevades the ultra-thin layer of the tape. Moreover, since it was developed for, and is sold to, a specific audio recording market, it is clearly not intended for, nor tolerant of any substantial bimodal character.
In the following description, the term "card" will be used in the same sense of a mechanical equivalent of any article which carries magnetic material in information-yielding arrangement. It will be obvious that the scope of the kinds of members which can be tagged with magnetic identification means is very broad and that such recording members are mechanical equivalents to the credit card referred to in this application.
Also, U.S. Pat. Nos. 3,986,205 and 3,986,206 to Fayling, based on applications filed subsequently to the parent application of Applicant, describe a special mode of bimodality whereby particular magnetic particles such as barium ferrite are utilized in a particular way to form bimodal materials. Barium ferrite is a peculiar material exhibiting a very high coercivity in one direction in comparison to the coercivity exerted in a direction normal thereto. The coercivity ratio is about 2.5:1 whereas the ratio in most magnetic materials is below 1.5:1 and usually about 1.25:1. This large difference in coercivity allows Fayling, using special processing procedures, to achieve a kind of polymodal system, or so these patents indicate.
Fayling utilizes his barium ferrite and like materials as one component of a polymodal system. Faylings's polymodal recording media cannot be processed in the usual way, i.e. using a single ring type erase-head of the type normally used in the magnetic recording art. This is true with respect to tape processing. Moreover, a spatially large erase field must be used in processing a credit card-sized object by the Fayling system.
Finally, Fayling's system does not appear to be useful on magnetic discs without the development of wholly new processing procedures.
It should also be noted that both operable powder populations of Fayling are believed to exhibit shape anisotropy. This is a consequence, it is believed, of chip-like barium ferrite flakes (normally not having shape anisotropy) aligning themselves into a "stack" formed of a plurality of the chips when they are mixed and oriented in a polymer matrix.
U.S. Pat. No. 3,566,356 to Holm et al discloses some magnetic recording media using two magnetic materials in the same composition. However, the purpose of these inventions is to provide a positive interaction between the two populations to assure a characteristic and distinctive coercivity characteristic. Holm et al also appear to disclose some two-layer particulate magnetic materials which seem to be capable of some bimodal performance. This Holm et al bimodality requires a difference in coercivity but the characteristic is achieved by cross-orienting two layers of iron oxide.